In general, larger L2 caches are preferred. We have found that a CPU with a larger has better Java performance than one of the same speed with a smaller cache.
But the most important thing is to have plenty of RAM. 2GB is adequate. With that you can set the heap to 1200MB and the yound generation to 400MB, which is a decent starting size for performance tuning your app.
If you can afford it, go for dual processor. App servers tend to run a lot of threads and having twice the number of CPUs available to run the threads on makes a big difference.
Of course, the above recommendations are for getting the most performance out of JBoss. If you have only a handfull of users and don't expect a lot of hits, a single processor machine with 512MB of memory will do the job just fine.